1. Field of the Invention
The present invention relates to an acceleration sensor and a semiconductor device.
2. Description of Related Art
Loading of a sensor (MEMS sensor) employing an MEMS (Micro Electro Mechanical Systems) technique on a portable telephone has recently been started, whereby the MEMS sensor draws increasing attention. An acceleration sensor for detecting the acceleration of an object is known as a typical MEMS sensor.
FIG. 10 is a sectional view schematically showing the structure of a conventional acceleration sensor.
The acceleration sensor 101 shown in FIG. 10 includes a circuit chip 104 having a circuit for calculating and correcting acceleration, a sensor chip 105 having a piezoresistor (not shown) and a weight 106 of tungsten in a cavity formed of a ceramic package 102 and a shielding plate 103.
The ceramic package 102 has a six-layer structure obtained by laminating six ceramic substrates 102A to 102F, for example. The lower three ceramic substrates 102A, 102B and 102C have rectangular shapes of the same size in plan view. The upper three ceramic substrates 102D, 102E and 102F have the same outline as the ceramic substrates 102A, 102B and 102C in plan view, and are formed with rectangular openings on each central portion. The opening of the ceramic substrate 102D laminated on the ceramic substrate 102C is smaller than that of the ceramic substrate 102E laminated on the ceramic substrate 102D. The opening of the ceramic substrate 102E is smaller than that of the ceramic substrate 102F laminated on the ceramic substrate 102E.
A plurality of pads 107 are arranged on the upper surface of the ceramic substrate 102D. The respective pads 107 are electrically connected to the circuit chip 104 and the sensor chip 105 through bonding wires 108, respectively. Wires 109 extending from the pads 107 are formed on the upper surface of the ceramic substrate 102D. The respective wires 109 are connected to an electrode 111 arranged on the lower surface of the lowermost ceramic substrate 102A through via holes 110 vertically penetrate the lower three ceramic substrates 102A, 102B and 102C.
The shielding plate 103 is bonded to the upper surface of the uppermost ceramic substrate 102F to close the opening of the ceramic substrate 102F.
The circuit chip 104 is formed of a silicon chip. The circuit chip 105 is bonded to the upper surface of the ceramic substrate 102C through silver paste, while directing the front surface of a device forming region thereof upward.
The sensor chip 105 integrally includes a membrane 112, a frame-shaped support section 113 connected to a peripheral edge portion of one surface (lower surface) of the membrane 112 and a weight fixing section 114 connected to the central portion of the one surface of the membrane 112. The piezoresistor (not shown) is formed on the other surface (upper surface) of the membrane 112. The support section 113 and the weight fixing section 114 are separated from each other by an annular groove 116 having a section in the form of an isosceles trapezoid narrowing as approaching the membrane 112.
The sensor chip 105 is supported above the circuit chip 104 by chip spacers 115 interposed between the respective corner portions of the support section 113 and the front surface of the circuit chip 104 at a prescribed interval with respect to the front surface of the circuit chip 104.
The weight 106 is fixed to the lower surface of the weight fixing section 114 with an adhesive in a state not in contact with the circuit chip 104, the support section 113 and the chip spacers 115 between the circuit chip 104 and the sensor chip 105.
When acceleration acts on the acceleration sensor 101 and the weight 106 oscillates, the membrane 112 is deformed, and stress acts on the piezoresistor provided on the membrane 112. The resistivity of the piezoresistor changes in proportion to the stress acting thereon. Therefore, the acceleration acting on the acceleration sensor 101 can be obtained on the basis of the change in the resistivity of the piezoresistor.
In the conventional acceleration sensor 101, however, the cost is disadvantageously increased due to the employment of the ceramic package 102. Further, the weight 106 is provided independently of the circuit chip 104 and the sensor chip 105, whereby the acceleration sensor 101 is hard to downsize.
Further, when the weight 106 (membrane 112) oscillates, the membrane 112 is distorted, and the distortion may propagate to the peripheral edge portion of the membrane 112 to crease the peripheral edge portion.
The sensor chip 105 is formed by etching a silicon wafer from the rear surface side (opposite to the side provided with the membrane 112). Since a thin layer portion on the front surface of the silicon wafer must be left as the membrane 112, the silicon wafer cannot be etched from the front surface side. In a step of manufacturing the conventional sensor chip 105, therefore, the silicon wafer can be etched only from the rear surface side, and a long time is required for the etching.
When a wet etching process is employed for etching the silicon wafer, the silicon wafer is etched in the direction parallel to the rear surface thereof through the so-called side etching. In order to pattern the silicon wafer into a desired shape, therefore, the shape of a mask (resist pattern) formed on the rear surface of the silicon wafer must be decided in consideration of both of the etching rate in the direction perpendicular to the rear surface of the silicon wafer and the rate of the side etching. Therefore, much labor is required for designing the mask.
Further, steps of manufacturing an MEMS sensor include a step of thinning a silicon wafer. The silicon wafer can be thinned by grinding and/or etching the silicon wafer from the rear surface side (opposite to the front surface formed with a device). At present, a silicon wafer employed for an MEMS sensor is thinned up to a thickness of about 200 μm to 400 μm in a wafer thinning step.
As shown in FIG. 11, an edge portion of a silicon wafer has a round (convexed) sectional shape. If the silicon wafer is thinned to not less than ½ of the original thickness, therefore, the sectional shape of the edge portion is sharpened, and the mechanical strength of the edge portion is extremely reduced. Consequently, the edge portion of the silicon wafer may be broken by the so-called edge chipping when the silicon wafer is handled.